1. Field of the Invention
This invention relates to an improved method and apparatus for producing seamless tubular articles, from extrudable thermoplastic resin, that have an integral bend or arcuate portion along the longitudinal axis of the tube. The method and apparatus may be used with particular advantage to mold tubular articles which include a first arcuate section leading into another arcuate section having a different radius than or curved in a direction opposite to the first. For example, plumbing fixtures such as traps and related conduits are characterized by these arcuate configurations.
Moreover, since plastic plumbing fixtures offer certain advantages like better resistance to corrosion and reduced material and production costs, they are being installed instead of similar metal fixtures in many newly constructed buildings.
2. Description of the Prior Art
Plastic injection molding is one technique which has been used for making tubular plastic articles such as plumbing fixtures. However, injection molding can only be used practically to manufacture articles such as elbows and tees having open ends relatively close coupled to the curved or bent section, or to make other articles having a shape which permits easy withdrawal of a mold core that defines the tubular interior. An example of this technique is shown in U.S. Pat. No. 3,092,441 (Bilderbeek).
Fixtures such as J-shaped traps which include an arcuate section that leads into a straight section along the tubular axis are difficult to produce using injection molding techniques. An ordinary solid mold core becomes encased in the finished product since the bend in the article obstructs its removal. Sectional or articulated cores, such as those shown in U.S. Pat. Nos. 3,028,630 (Walker) and 3,200,023 (Cilker), have been proposed to solve this problem. However, the devices used in such methods are complex and the shapes of articles which may be fabricated by them are limited, making it impossible to meet established dimensioned standards of systems based on the prior metal counterparts.
Other proposals contemplate molding complex shapes from highly elastomeric plastics which may be greatly deformed to permit removal of the mold core. However, the finished product remains non-rigid and flexible and, accordingly, is difficult to connect with other fixtures in a liquid or gas-tight manner. Examples of flexible molded articles are shown in U.S. Pat. Nos. 2,484,031 (Havrenius) and 1,536,700 (Brucker).
Blow molding is another technique used in the past for making plastic articles. U.S. Pat. No. 3,291,670 (Usab) discloses a method where a closed parison is extruded vertically downward into an open mold. The mold is subsequently clamped about the parison which is blown outwardly to conform to the shape of the mold's inner surface. However, this technique is not well suited for molding articles having complex arcuate shapes because various parts of the parison which must reach both remote and nearby regions of the mold expand non-uniformly and create article walls of varying thicknesses.
U.S. Pat. No. 3,719,209 (Rush et al), assigned to the assignee of the present application, discloses a method for manufacturing arcuate tubular articles of rigid plastic by extruding a large diameter parison between mating die members. The die members are clamped together folding the parison onto itself to be heat welded along a seam. A blow molding duct is introduced into the parison and die at a point adjacent one end of the article to be formed and fluid, under pressure, is introduced through the duct to force the parison wall into contact with the mold cavity surface. However, a weakness can result at the seam. Further, substantial amounts of flash result which must be trimmed and recycled to again be molded.
Other blow molding techniques are disclosed in U.S. Pat. Nos. 3,452,125 (Schurman et al), 3,412,187 (Fogelberg et al), 3,300,556 (Battenfield et al), 3,100,317 (Perry), 2,983,961 (Titterton et al), 2,975,472 (Colombo) and 2,897,840 (Roberts et al).